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Transpower will investigate why five contractors, who were at the centre of an emergency in an Auckland tunnel last night, were not able to radio back to their employers for three hours.
The Transpower staff had been carrying out maintenance in Auckland and were to report back about 7.30pm but failed to do so.
They were not found until 10.45pm when firefighters entered a substation tunnel on Nuffield St in Newmarket.
Part of the street was cordoned off as fire crews - some with breathing apparatus - from around the city gathered.
Ambulances were also on standby.
The 3km tunnel ends at Hobson St in the city, where Fire Service staff were also on standby.
The contractors would normally check-in at 30-minute intervals.
Transpower chief executive Patrick Strange told Radio New Zealand that the tunnel was very dry and inert so controllers suspected there had been a communication failure, but had to assume the worse.
"They should have realised, but we'll look into all of that because it requires a positive two-way communication," Mr Strange said.
"But something's happened obviously which means the [communications] failed, we can't wait around to assume the [communications] failed. We get everyone here just in case."
It is understood that Mines Rescue staff from Huntly, who were among the personnel involved in the aftermath of the Pike River tragedy, were on standby to assist.